AST 105 Intro Astronomy The Solar System MIDTERM II: Tuesday, April 5 [covering Lectures 10 through 16]
REVIEW Light as Information Bearer We can separate light into its different wavelengths (spectrum). By studying the spectrum of an object, we can learn its: Composition Temperature Velocity
The Electromagnetic Spectrum REVIEW 400-700 nm
REVIEW Wave-Particle Duality of Light Light can behave like a wave Frequency, wavelength, amplitude Light can also behave like a particle Photons = little bundles (bullets) of energy
Light as a WAVE REVIEW Wavelength is the distance between peaks All light travels with a constant speed λ x f = c Frequency is the number of times (per second) that the wave moves up and down OR f = c / λ OR λ = c / f The shorter the wavelength, the higher the frequency
Clicker Question Infrared light with a wavelength of 3 microns (3x10-6 m) has a frequency of 1.0x10 14 Hz. What is the wavelength of light that has a frequency of 0.5 x10 14 Hz? (Hint: What is the relationship between wavelength and frequency?) A. 1.5 microns B. 2.0 microns C. 2.5 microns D. 3.5 microns E. 6.0 microns
Clicker Question Infrared light with a wavelength of 3 microns (3x10-6 m) has a frequency of 1.0x10 14 Hz. What is the wavelength of light that has a frequency of 0.5 x10 14 Hz? (Hint: What is the relationship between wavelength and frequency?) A. 1.5 microns B. 2.0 microns C. 2.5 microns D. 3.5 microns E. 6.0 microns
Light as a PARTICLE Light can also be modeled as a particle photon A photon is a massless particle of electromagnetic radiation energy Photons travel at, you guessed it, the speed of light, 300,000 km/s
Each photon has a unique energy E = h x f Photon Energy Planck s = Constant x Frequency
Each photon has a unique energy proportional to its frequency E f E 1 / λ Remember: means is proportional to Higher Frequencies or Shorter Wavelengths MORE ENERGY Gamma rays, X-rays, & UV are more dangerous than visible, infrared, or radio waves
Clicker Question When compared to RED light, Blue light is: A. Longer wavelength B. Higher Frequency C. Lower energy photons D. Faster photons E. None of the above
Clicker Question When compared to RED light, Blue light is: A. Longer wavelength B. Higher Frequency C. Lower energy photons D. Faster photons E. None of the above
Light as Information Bearer We can separate light into its different wavelengths (spectrum). By studying the spectrum of an object, we can learn its: Composition Temperature Velocity
First, a quick review of atoms
Electron Energy Levels in Atoms Atoms are made up of protons(+), neutrons (0) and electrons(-) e - Electrons move in different energy states around the nucleus Some states have more energy than others. These energy states are quantized there are only certain energies that the electrons are allowed to have. Details contained in quantum physics.
Example of electron energy levels in a hydrogen atom Lower level is lower energy. (think of stairs)... Units: electronvolt (ev) TINY! 1 Calorie = 3 x 10 22 ev
Two ways to look at energy levels... level 3 level 2 level 1 Both represent the same thing The higher (or further from the nucleus) the level is, the more energy that level has.
Electrons can move between levels if they are given or lose the exact amount of energy corresponding to the difference in the energy levels.... E F If an electron gets enough energy, the electron will fly free Example: Energy jumps A & C result in the electron losing energy, B & F require energy, and D & E are not possible. F ionizes the atom with an energy gain of 3.4 ev
When an electron drops down a level, it releases energy. Where does that energy go? The energy change between levels is equal to the energy of the photon. Larger energy jumps will be SHORTER wavelength photons PHOTONS!
Clicker Question What causes spectral lines? A. Blackbody radiation. B. Electron energy level transitions in an atom. C. The Doppler shift of rapidly moving objects. D. High frequency electromagnetic waves. E. Protons and neutrons spinning in a charged atom.
Clicker Question What causes spectral lines? A. Blackbody radiation. B. Electron energy level transitions in an atom. C. The Doppler shift of rapidly moving objects. D. High frequency electromagnetic waves. E. Protons and neutrons spinning in a charged atom.
Three types of spectra
Emission Spectra Emission for thin, hot gas: Gas glows in specific colors.
Clicker Question Why does it need to be a HOT gas to give off an emission spectrum? A. The electrons need to be in high energy levels B. Hot gases give off higher energy photons C. Hot gases glow brighter than cold gases D. Cold photons don't have enough energy to make it here to Earth E. Hot things glow, cool things don t.
Clicker Question Why does it need to be a HOT gas to give off an emission spectrum? A. The electrons need to be in high energy levels B. Hot gases give off higher energy photons C. Hot gases glow brighter than cold gases D. Cold photons don't have enough energy to make it here to Earth E. Hot things glow, cool things don t.
Emission Spectra Emission for thin, hot gas: Gas glows in specific colors. Colors represent electrons falling down energy levels This is a FINGERPRINT of the elements in the gas.
Each atom has a different set of energy levels Just like no two people have the same fingerprints, no two elements have the same emission spectrum
The Crab nebula: remains of an exploded star (supernova) Spectrum shows bright emission lines from various elements
Continuous Spectrum Hot solids (or dense liquid): Emit a continuous rainbow of light Thermal Radiation (or Blackbody Radiation)
Three types of spectra
Absorption Spectrum Hot object viewed through COOL gas: Dark lines on top of a rainbow Gas can only absorb photons OF THE RIGHT ENERGIES to move electrons to excited states
Absorption/Emission Line Applet http://spiff.rit.edu/classes/phys301/lectures/spec_lines/atoms_nav.swf
1) Hot solid, liquid, or dense gas Kirchhoff's Laws 2) Thin, hot gas (compared to background) 3) Continuous spectrum viewed through a cooler gas (compared to 1 2 3 background)
Solar Spectrum (as seen from Earth) 34
What color are hot objects? Classic example: red hot pokers: As temperature increases: IRà red à blue
Colors of Hot, Solid Objects Hotter objects peak at bluer wavelengths (photons with a shorter wavelength, higher frequency, and higher average energy.) Wien's Law λ Peak 1 / T We won't worry about the other mathematical law presented in the book: the Stefan-Boltzmann law
Clicker Question The three spectral curves shown in the graphs below illustrate the total energy output (over the whole surface) versus wavelength for three unknown objects. Which of the objects has the highest temperature? Total energy output A UV VIBGYOR Infrared Wavelength Total energy output UV VIBGYOR B Wavelength Infrared Total energy output UV VIBGYOR C Wavelength Infrared
Clicker Question The three spectral curves shown in the graphs below illustrate the total energy output (over the whole surface) versus wavelength for three unknown objects. Which of the objects has the highest temperature? Total energy output A UV VIBGYOR Infrared Wavelength Total energy output UV VIBGYOR B Wavelength Infrared Total energy output UV VIBGYOR C Wavelength Infrared
Humans in the Infrared (false color)
What can a spectrum tell us? Let's use its spectral information to determine what this object is.
What is this object? Continuous Spectrum: Spectrum of visible light is like the Sun's except that some of the blue light has been absorbed
What is this object? Continuous Spectrum: Must be a solid object with peak emission at a wavelength corresponding to a temperature of 225 K
What is this object? Infrared Absorption Lines: Absorption lines are the fingerprint of CO 2 gas
What is this object? Ultraviolet Emission Lines: Indicate object is surrounded by a hot upper layer of gas
What is this object? Mars!